MTBF unit was the measurement of the reliability of a hardware component. MTBF was an abbreviation for mean time between failures.
In 2367, when Doctor Leah Brahms was reviewing modifications to the engines of the USS Enterprise-D, she noted that Chief Engineer Geordi La Forge had completely charted a new swap-out schedule for main component replacements. La Forge responsded to her observation by stating he found Starfleet estimates of MTBF units were unrealistic. He made a new schedule based on observation and experience. (TNG: "Galaxy's Child")